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Question about gains ?

Discussion in 'Electronic Basics' started by philips, Jun 21, 2004.

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  1. philips

    philips Guest

    Gain is usually V2/V1, but sometimes it's I2/I1, or even V2/I1
    (also known as Z21) I was simply wondering when it is appropriate
    for gain to be V2/V1 or I2/I1 etc i.e. is it more informative
    to talk about voltage gain than current gain, is it hourses for
    courses?
     
  2. It depends on what is important in the circuit. If it is a signal
    amplifier and the signals are represented by voltages (as in common
    with audio) and input impedance is assumed to be much higher than the
    impedance of the source feeding the stage, and the output impedance of
    the stage is much lower than its load, voltage gain is the natural way
    to look at the stage. If the stage is an RF gain stage that needs to
    match source and load impedances, then voltage gain and current gain
    are related by the pair of impedances, so voltage gain, current gain ,
    Vout/Iin, Iout/Vin, or most useful, PowerOut/PowerIn are all
    reasonable. If the voltage gain is nearly 1 as with all followers,
    voltage gain is not nearly as useful a way to describe the stage as
    current gain.

    Am I addressing your question or did it go right by me?
     
  3. CFoley1064

    CFoley1064 Guest

    Subject: Question about gains ?
    Good question. Horses for courses. If you're talking about a standard op amp,
    you usually talk about voltage gain. For a current-controlled device like a
    standard transistor, you usually talk about current gain. For amplifiers, you
    frequently talk about power gain. All three are common, but it's always
    apples2/apples1 or oranges2/oranges1. V2/I1 might measure impedance, but it
    doesn't quantify gain. Gain of any kind is a dimensionless quantity, which
    means the terms have to cancel.

    Gain is sometimes quantified logarithmically in decibels. Here's a good basic
    tutorial on decibels from the National Instruments Developer Zone:

    http://zone.ni.com/devzone/conceptd.nsf/webmain/36EA112B7F683D768625681100
    4DD454

    Good luck
    Chris
     
  4. philips

    philips Guest

    Hi, thanks for the reply. I have another question (maybe slightly
    more complicated). When we learnt about Network Synthesis, the
    lecturer always talked about transfer functions. Is gain and
    transfer functions the same thing. Also, a lot of the time my
    notes refer to Z21 transfer function, is this gain too, i.e.
    V2/I1
     
  5. Gains described as V/I have units of ohms, usually referred to as
    transresistance. Gains of I/V have units of siemens (reciprocal ohms)
    usually referred to as transconductance. You have to combine these
    ratios with source or load impedances to make much sense of them as
    gain. A transformer can produce a wide variety of transresistances or
    transconductances in a given circuit but none of them provide power
    gain.
     
  6. JeffM

    JeffM Guest

    [gain is] always apples2/apples1 or oranges2/oranges1.
    Probably because he meant I2/V1 (transconductance).
     
  7. Bob Myers

    Bob Myers Guest

    Nope; if I2/V1 is a valid sort of "gain" (and it is), then
    so is V2/I1 (transresistance, as John said - the shortened
    version of "transfer resistance"). And just what do you
    think is the origin of the name "transistor"?

    Just because you're not familiar with "gain" being expressed
    in ohms doesn't mean it's not a valid thing to do - and actually,
    pretty useful in the proper context.

    Bob M.
     
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